In the past few years, there has been considerable interest in using optical scatterometry (i.e., diffraction) for performing critical dimension (CD) measurements of the lines and structures that comprise an integrated circuit. It has been shown that optical scatterometry from periodic two-dimensional structures (e.g. line gratings) and from three-dimensional structures (e.g. patterns of vias or mesas) can provide accurate and precise measurements of the spatial profiles of these various lines, vias and mesas. Various optical techniques can be utilized to perform optical scatterometry. These include broadband spectroscopy (U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) single-wavelength optical scattering (U.S. Pat. No. 5,889,593), and spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943) (all incorporated herein by reference). In addition it may be possible to employ single-wavelength laser BPR or BPE to obtain CD measurements on isolated lines or isolated vias and mesas. To obtain critical dimension measurements from optical scatterometry signals, one generally employs some rigorous optical diffraction models. As described in above cited patents, these models are then used to generate look-up libraries of optical responses for various combinations of critical dimensions and material properties, or the models are used directly in a real-time iterative process. The above cited patents and patent applications, along with PCT Application WO03/009063, U.S. Application 2002/0158193, U.S. application Ser. No. 10/243,245, filed Sep. 13, 2002, U.S. Application 2001/0051856 A1, PCT Application WO 01/55669 and PCT Application WO 01/97280 are all incorporated herein by reference.
More recently, a new application has emerged for optical scatterometry, overlay registration (overlay) measurements. In overlay measurements one desires to determine with high precision how accurately successive lithographic mask layers have been aligned to preceding layers. Optical scatterometry appears to show considerable promise for performing such measurements using some or all of the techniques described above for CD measurements, with appropriate modifications in the model and analysis.
However, a significant problem for both CD and overlay measurements with optical scatterometry is the problem of asymmetric lines. Asymmetric CD lines (or structures such as vias or mesas) may be the result of stepper artifacts, or from photoresist stripping or material etching processes. FIG. 1 illustrates a simple asymmetric line pattern where the right side wall angle a is smaller than the left side wall angle β Asymmetric overlay lines or patterns are a natural consequence of layer-to-layer misalignment. The problem of asymmetric lines is that in most cases it is not possible to distinguish between right and left asymmetries with conventional optical scatterometry techniques without resort to special or multiple patterns. Discussions related to the use of grating like patterns to evaluate mask overlay can be found in U.S. Pat. Nos. 4,757,207; 6,023,338 and 6,079,256, and U.S. 2002/0158193 all incorporated herein by reference.
It would desirable to be able to evaluate asymmetries in periodic structures. Obtaining such information would be useful in analyzing basic process parameters for fabricating integrated circuits having two dimensional and three dimensional structures (for example, grating line patterns and vias). Such a capability would also be useful to determine in which direction an overlay error lies.